On 11/01/2017 04:53 PM, Andrew Morton wrote: > On Tue, 31 Oct 2017 07:58:21 -0700 Davidlohr Bueso <[email protected]> wrote: > >> On Tue, 31 Oct 2017, Jason Baron wrote: >> >>> The use of ep_call_nested() in ep_eventpoll_poll(), which is the .poll >>> routine for an epoll fd, is used to prevent excessively deep epoll >>> nesting, and to prevent circular paths. However, we are already preventing >>> these conditions during EPOLL_CTL_ADD. In terms of too deep epoll chains, >>> we do in fact allow deep nesting of the epoll fds themselves (deeper >>> than EP_MAX_NESTS), however we don't allow more than EP_MAX_NESTS when >>> an epoll file descriptor is actually connected to a wakeup source. Thus, >>> we do not require the use of ep_call_nested(), since ep_eventpoll_poll(), >>> which is called via ep_scan_ready_list() only continues nesting if there >>> are events available. Since ep_call_nested() is implemented using a global >>> lock, applications that make use of nested epoll can see large performance >>> improvements with this change. >> >> Improvements are quite obscene actually, such as for the following >> epoll_wait() >> benchmark with 2 level nesting on a 80 core IvyBridge: >> >> ncpus vanilla dirty delta >> 1 2447092 3028315 +23.75% >> 4 231265 2986954 +1191.57% >> 8 121631 2898796 +2283.27% >> 16 59749 2902056 +4757.07% >> 32 26837 2326314 +8568.30% >> 64 12926 1341281 +10276.61% >> >> (http://linux-scalability.org/epoll/epoll-test.c) > > This is tempting, but boy it is late in the -rc cycle. > > How important are these workloads? Would the world end if we held off > on this for 4.15? >
Hi Andrew, As Davidlohr pointed out, the nested epoll case is less common and these locks have been here for a long time. I also think the patch needs to be in linux-next for a bit and validated more, before hitting mainline. Thanks, -Jason
